A steam generator provided in a nuclear plant has an inspection hole formed on an outer casing that surrounds the tube bundle including heat-transfer tubes to perform internal checkup or inspection during plant suspension. The inspection hole is plugged by the inspection hole plug during plant operation, so that a secondary coolant does not circulate inside or outside the outer casing through the inspection hole.
Conventionally, an inspection hole plug (a wrapper closure assembly) described in Patent Literature 1 includes a plate fitted to an outer casing of a tube bundle, a plug having a first end for plugging a bore penetrating the plate, a sleeve into which a second end of the plug is inserted, and a nut attached to the second end of the plug. The plate includes a connection on an inner circumference of the bore. At least one aperture is formed at the connection. The plug includes a flange fitted to the connection at the first end. The flange includes an aperture aligned with the aperture in the connection when the bore is plugged by the first end of the plug. The sleeve includes a pin installed through the aligned apertures of the plate and the plug. When the nut is attached to the second end of the plug, the pin of the sleeve is prevented from slipping out of each aperture, thereby fixing the plug to the plate.